Combined electron microscopy and immunocytochemistry of the larvae of several polyplacophoran species (Chiton olivaceus, Lepidochitona aff. corrugata, Mopalia muscosa) revealed a sensory system new to science, a so-called "ampullary system." The cells of the "ampullary system" are arranged in four symmetrically situated pairs lying dorsolaterally and ventrolaterally in the pretrochal part of the trochophore-like larva and they send axons into the cerebral commissure. They are lost at metamorphosis. The fine structure of these cells strongly resembles that of so-called "ampullary cells" known from various sensory organs of other molluscs, such as the apical complex of gastropod and bivalve larvae, osphradia of vetigastropods, and olfactory organs of cephalopods, and nuchal organs of certain polychaetes. The ampullary cells and their nerves are densely stained by anti-FMRF-amide fluorescence dyes, whereas antiserotonin staining is only weak. While cytological homology of the ampullary cells with those of other organs is probable, the ampullary system as a whole is regarded as a synapomorphy of the Polyplacophora or Chitonida.